Digital fluid flow rate measurement or control system

ABSTRACT

A plurality of individually actuatable, value weighted digital bistable valve elements in parallel interconnect a fluid source to a fluid receiver. A linear relationship is preferably maintained between the resultant fluid flow rate from the source to the receiver and the product of a flow rate determinative fluid parameter times the sum of the weighted values of the digital valve elements in the open state. The fluid parameter is sensed, the states of the digital valve elements are controlled, and a flow rate representative signal is derived from the states of the valve elements and the fluid parameter. If the fluid is liquid, the parameter is the square root of the pressure difference across the valve elements, in the absence of cavitating venturis, and is the square root of the difference between the upstream pressure and the vapor pressure of the liquid in the presence of cavitating venturis. If the fluid is gas, the parameter is the source pressure divided by the square root of the source temperature. For measurement, the states of the digital valve elements are controlled to maintain the value of the fluid parameter constant. For control the states of the digital valve elements are controlled to establish a set point flow rate.

BACKGROUND OF THE INVENTION

This invention relates to the measurement or control of fluid flow rateand, more particularly, to the application of digital techniquesthereto.

Conventionally, fluid flow rate is measured by a flowmeter, such as aventuri meter, an orifice meter, or a turbine meter. In a venturi meterand an orifice meter, the flow rate is proportional to the pressure in afluid passage having fixed cross-sectional dimensions. In a turbinemeter, the flow rate is proportional to the angular velocity at whichthe turbine rotates. For any particular meter, flow rate is proportionalto the measured parameter within a limited range of flow rates.Therefore, to make accurate measurements over a wide range of flowrates, a number of particular meters having different dimensions mustoften be employed, each covering a segment of the range.

In a conventional analog fluid flow control system, the flow rate iscontrolled by positioning a plug located in the fluid stream. The degreeto which the plug impedes flow governs the flow rate. In order toestablish a set point value of flow rate, a flowmeter generates a signalrepresentative of the actual value of flow rate, which is compared witha command signal representative of the set point value, and the plugposition is adjusted by a control loop until the actual valuecorresponds to the set point value. In large oil refineries, chemicalplants, and other processing facilities, supervisory digital computersrun the operations by issuing set point commands to the individual flowcontrol systems and receiving data concerning the status of theoperations. The limited range of present flowmeters mentioned in thepreceding paragraph, however, restricts the range of set point valuesthat an analog fluid flow control system can accurately accommodate.Further, it is difficult to derive the actual value of flow rateindirectly by calculation because flow rate depends in part on theeffective cross-sectional area of the flow passage which is a complexfunction of the plug position.

In a digital fluid flow control system, a plurality of individuallyactuatable, value weighted digital bistable valve elements in parallelinterconnect an upstream manifold to a downstream manifold. Each valveelement exclusively assumes either an open state in which fluid flowsfrom the upstream manifold through the valve element to the downstreammanifold, or a closed state in which no fluid flows from the upstreammanifold through the valve element to the downstream manifold. Theeffective cross-sectional orifice areas of the flow passages through therespective valve elements are weighted according to a binary code, e.g.a geometric progression of two, thereby value weighting the digitalvalve elements. The valve elements are actuated by binary signalsweighted according to the same binary code as the respective valveelements to which they are coupled. The sum of the effective orificeareas of the valve elements in the open state is related to the binarynumber or value represented by the actuating signals in the binary code.Recent improvements in the design of digital fluid flow control systemshave virtually eliminated any interaction between valve elements, i.e.,any dependence of the effective orifice area of one valve element uponthe states of the other valve elements, and minimized the effect ofpressure variations and ambient conditions on effective orifice areas.Consequently, the sum of the effective orifice areas of the open valveelements can be made proportional to the binary number represented bythe binary actuating signals to a high degree of accuracy.

SUMMARY OF THE INVENTION

According to the invention, a digital fluid flow control system isemployed to measure or control fluid flow rate. A plurality ofindividually actuatable, value weighted digital bistable valve elementsin parallel interconnect an upstream manifold to a downstream manifold.Each valve element exclusively assumes either an open state in whichfluid flows from the upstream manifold through the valve element to thedownstream manifold, or a closed state in which no fluid flows from theupstream manifold through the valve element to the downstream manifold.The states of the valve elements comprise a binary number representativeof the total effective orifice area between the upstream and downstreammanifolds, i.e., the sum of the weighted values of the valve elements inthe open state. Preferably, means are provided to maintain a linearrelationship between the resultant fluid flow rate from the upstreammanifold to the downstream manifold and the product of a flow ratedeterminative fluid parameter times the sum of the weighted values ofthe digital valve elements in the open state. The fluid parameter issensed and the states of the digital valve elements are controlled, anda flow rate representative signal is derived from the states of thevalve elements and the fluid parameter. In the preferred embodiments,the states of the digital valve elements are controlled partially orwholly responsive to the sensed fluid parameter. The invention may beviewed as functioning as an orifice meter having a plurality ofdifferent size orifice plates corresponding to the differentpossibilities of the sum of the weighted values of the digital valveelements; responsive to the sensed fluid parameter, the "orifice plate"with the appropriate size orifice is selected. The flow rate range ofthe system can be increased without impairing accuracy by simply addingmore valve elements.

If the fluid is incompressible, the sensed parameter is the square rootof the pressure difference between the upstream and downstreammanifolds. The linear relationship is maintained by establishing asufficiently low maximum pressure difference to avoid vena contractaeffects or by dissipating the vena contractas. In the special case wherea cavitating venturi is provided in the flow passage through each valveelement to maintain the linear relationship, the sensed parameter is thesquare root of the difference between the pressure in the upstreammanifold and the vapor pressure of the fluid.

If the fluid is compressible, the sensed parameter is the absolutepressure in the upstream manifold divided by the square root of theabsolute temperature in the upstream manifold. The linear relationshipis maintained by establishing a minimum pressure difference that issufficiently large so fluid passes through the flow determining orificesof the open valve elements at sonic velocity.

For the preferred embodiment of flow rate measurement, the valveelements are wholly controlled responsive to the fluid parameter suchthat the valve of the sensed fluid parameter remains constant. The valueof the measured flow rate is related to the states of the valueelements, i.e., the sum of the cross-sectional areas of the valveelements in the open state. Therefore, the binary actuating signals andthe constant fluid parameter are multiplied to derive a signalrepresentative of the value of the measured flow rate, which can bedisplayed by a digital indicator. Accurate measurement over a wide rangeof flow rates can be accomplished in this manner.

For the preferred embodiment of flow rate control, the valve elementsare partially controlled responsive to the fluid parameter so thatactual flow rate equals a set point flow rate. The value of the actualflow rate is related to the product of the sensed fluid parameter timesthe states of the value elements, i.e., the sum of the effective orificeareas of the valve elements in the open state. Therefore, the binaryactuating signals and the signal representative of the sensed fluidparameter are multiplied to derive a signal representative of the valueof the actual flow rate. In this manner, a set point value of flow ratecan be accurately established by a control loop over a wide range offlow rates with a minimum of sensing transducers and without complexcomputations.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of specific embodiments of the best mode contemplated ofcarrying out the invention are illustrated in the drawings, in which:

FIGS. 1A, 1B and 1C are schematic diagrams of different embodiments of adigital fluid flow rate measurement or control system incorporating theprinciples of the invention;

FIG. 2 is a schematic diagram of the control circuitry shown in FIGS.1A, 1B and 1C;

FIG. 3 is a schematic diagram of the sequence circuit shown in FIG. 2;

FIGS. 4A, 4B and 4C are schematic diagrams of the local computer shownin FIG. 2 for the embodiments of FIGS. 1A, 1B and 1C, respectively;

FIG. 4D is a schematic diagram of an alternative version of a portion ofthe local computer shown in FIG. 2 for the embodiment of FIG. 1C; and

FIG. 5 is a schematic diagram of an arrangement for monitoring theoperation of the disclosed digital fluid flow rate measurement orcontrol system.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Reference is made to FIG. 2 for a schematic block diagram of controlcircuitry used to practice the invention. One or more analog signalsrepresentative of a flow rate determinative fluid parameter are coupledby a transmission gate 10 to an input terminal 20 of a local computer11. A pulse source 12 controls transmission through gate 10. Each timesource 12 generates a pulse, the parameter representative analog signalor signals are transmitted to local computer 11. Thus, the value of theparameter is sampled and applied to the input of computer 11 at a ratedetermined by the frequency of source 12. The heavy broken lead lines inFIG. 2, each represent a plurality of binary signal leads. For thepurpose of illustration, it is assumed that each heavy broken lead lineand each terminal and switch contact associated therewith representsfour binary signal leads weighted according to a binary code comprisinga straight geometric progression of 2 i.e., weighted 1, 2, 4 and 8. Inpractice, there would most likely be many more than four binary signalleads and any binary code could be used. An output terminal 24 ofcomputer 11 (representing four binary signal terminals) is coupledthrough a contact A of a switch SW-1 (representing four binary signalcontacts) to a first input of a digital comparator 13 and is coupledthrough a contact D of switch SW-1 to a digital indicator 14. Anintermediate output terminal 21 of computer 11 is coupled through acontact F of switch SW-1 to a second input of comparator 13. Asupervisory computer 15 is coupled through a contact G of a switch SW-2and a contact E of switch SW-1 to the second input of comparator 13 andis coupled through a contact M of switch SW-1 to an intermediate inputterminal 22 of computer 11. Computer 15 is also coupled through contactG of switch SW-2 and contact C of switch SW-1 to indicator 14. Usuallythe supervisory computer is remotely located from the measurement andcontrol system and oversees the operation of a number of differentprocesses. A digital slewing circuit 16 is coupled through a contact Hof switch SW-2 and contact C of switch SW-1 to indicator 14, is coupledthrough contact H of switch SW-2 and contact M of switch SW-1 tointermediate input terminal 22 of computer 11, and is also coupledthrough contact H of switch SW-2 and contact E of switch SW-1 to thesecond input of comparator 13. Circuit 16 could be a four stage counterdriven by a pulse source so its four binary output signals continuouslystep through the 16 states representing each value of the binary code insuccession. The output of an adjustable analog signal source 17 isconnected to an analog-to-digital converter 18. The output ofanalog-to-digital converter 18 is coupled through a contact B of switchSW-1 to the first input of comparator 13 and is coupled through acontact N of switch SW-1 to input terminal 22 of computer 11. The outputof comparator 13, which indicates whether the value of the binarysignals at its first input or the value of the binary signals at itssecond input is larger, is connected to a sequence circuit 19. Thebinary output signals produced by sequence circuit 19 are coupled to aninput terminal 23 of computer 11 and to the valve elements of a digitalfluid flow rate measurement or control system described below.

A schematic block diagram of sequence circuit 19 is depicted in FIG. 3.The output of a pulse source 25 is connected to one input of each of ANDgates 26 and 27. Output terminals 28 and 29 of comparator 13 areconnected to the other input of AND gates 26 and 27, respectively. Theoutput of AND gate 26 is coupled to an upcounting lead U of a reversiblecounter 30. The output of AND gate 27 is connected to a downcountinglead D of counter 30. Counter 30 has four binary stages connected so itsfour binary output signals step through the 16 states representing eachvalue of the binary code in succession responsive to respective steppingpulses. When the value represented by the binary signals applied to thefirst input of comparator 13 is larger than the value represented by thebinary signals applied to the second input of comparator 13, outputterminal 29 of comparator 13 is energized and pulses from source 25 areapplied by AND gate 27 to lead D of counter 30 to reduce the valuerepresented by the binary output signals of counter 30. Conversely, whenthe value represented by the binary signals applied to the second inputof comparator 13 is larger than the value represented by the binarysignals applied to the first input of comparator 13, output terminal 28of comparator 13 is energized and the pulses from source 25 are appliedby AND gate 26 to input U of counter 30 to increase the valuerepresented by the binary output signals of counter 30.

The operation of the circuitry of FIGS. 2 and 3 is controlled by pulsesource 12, which governs the frequency of the samples supplied to localcomputer 11. Each time a new sample is supplied to computer 11, a newvalue of flow rate is calculated and sequence circuit 19 assumes a newstate. The frequency of source 12 is selected to be sufficiently higherthan the rate at which the value of the set point flow rate fromsupervisory computer 15 varies when the system is controlling flow rate,to permit the circuitry to follow changing set point values, and thefrequency of source 12 is also sufficiently high when the system ismeasuring to give the desired response time. The frequency of pulsesource 25 is substantially higher than that of pulse source 12 so thatsequence circuit 19 is capable of stepping through all of the 16 statesbetween sampling periods. The frequency of the pulse source drivingslewing circuit 16 is preferably variable so that a human operator cancontrol the slew rate.

In FIG. 1A is depicted a digital fluid flow rate measurement or controlsystem for an incompressible fluid, such as water. An upstream fluidmanifold 35 is interconnected by fluid flow passages 36, 37, 38 and 39to a downstream manifold 40. Plugs 41, 42, 43 and 44 are disposed inpassages 36, 37, 38, and 39, respectively, where they are eachpositionable in response to an electrical actuating signal exclusivelyin a first position in which the plug seals an orifice to prevent fluidflow through the passage, or a second position in which the plug unsealsthe orifice to permit fluid flow through the passage. Each passage andits related plug comprises an individually actuatable, digital bistablevalve element. The valve elements are value weighted, i.e., the ratio oftheir effective cross-sectional orifice areas are equal to the weightingof the respective binary electrical valve actuating signals in a binarycode. For the purpose of illustration, it is assumed that the binarycode is a straight geometric progression of 2, i.e., 1, 2, 4 and 8. Inpractice more valve elements would normally be used. Fluid is suppliedto manifold 35 by a source in the form of a conduit 45 and removed frommanifold 40 by a receiver in the form of a conduit 46. The fluid flowsin the direction of the solid arrows.

Although any configuration could be employed for manifolds 35 and 40 andthe digital valve elements interconnecting them, it is preferable toemploy one of the configurations disclosed in application Ser. No.111,945, filed Feb. 2, 1971, now U.S. Pat. No. 3,746,041; applicationSer. No. 169,930, filed Aug. 9, 1971, now U.S. Pat. No. 3,785,389, orthe application Ser. No. 432,153, filed on even date herewith by HarryFriedland and Addison W. Langill, Jr., all of which are assigned to theassignee of the present application. The disclosures of these threeapplications are incorporated herein by reference. The resultant fluidflow rate from upstream manifold 35 to downstream manifold 40 throughall the digital valve elements that are in the open state is expressedby the following equation: ##EQU1## where Q is the mass flow rate of theincompressible fluid, C_(v) represents the sum of the effective orificeareas of the open valve elements, ΔP is the difference in staticpressure between the fluid in manifolds 35 and 40, and SG is thespecific gravity of the fluid. The C_(v) of each individual valveelement is defined as the flow rate (Q) of water in gallons per minute(GPM) through such valve element with a pressure difference (ΔP) of onepsi; the C_(v) in equation (1) is the sum of the C_(v) 's of theindividual valve elements in the open state. The fluid flow ratedeterminative parameter is the square root of the pressure difference(ΔP). The linear relationship in equation (1 ) is maintained in eitherof two ways. First, there is established a sufficiently low maximumpressure difference (ΔP) to prevent formation of downstream venacontractas having pressure dependent cross-sectional areas. (Withreference to the water flow curve of FIG. 5 in a paper by Gordon F.Stiles entitled "Cavitational Tendencies of Control Valves For PaperPulp Service," which was presented at the 21st Annual Conference of theInstrument Society of America, Oct. 24-27, 1966, in New York, N.Y., themaximum pressure difference should be small enough to operate on thestraight portion of the curve for each digital valve element.) Second,if the maximum pressure difference is high enough to form downstreamvena contractas having pressure dependent cross-sectional areas then thevena contractas should be dissipated by directing the streams from thedifferent valve elements at each other, as taught in application Ser.No. 64,142, filed Aug. 3, 1970, the disclosure of which is incorporatedherein by reference. Thus, the linear relationship is maintained bypreventing dependence of C_(v) upon ΔP. By definition, the specificgravity of an incompressible fluid is a constant at a constanttemperature. In practice, the specific gravity of most liquids, whichare the fluids regarded as incompressible, does not vary substantiallyover a wide range of ambient temperature. Accordingly, the flow rate isproportional to the product of the square root of the pressuredifference between manifolds 35 and 40 times the binary numberrepresenting the sum of the effective orifice areas of the digital valveelements in the open state and the flow rate can be computed therefromfor a specified fluid. Upstream manifold 35 and downstream manifold 40are fluidically coupled to a differential pressure transducer 47, whichgenerates an electrical analog signal proportional to the pressuredifference between manifolds 35 and 40 P_(u) - P_(d). This signal iscoupled to control circuitry 49, which is discussed above and disclosedin FIG. 2 in detail. Control circuitry 49 generates binary outputsignals that represents the actual flow rate (Q) in the binary code toactuate plugs 41, 42, 43, and 44, respectively, partially or wholly inresponse to the sensed fluid parameter, i.e., the square root of P_(u) -P_(d).

If the specific gravity of the fluid in the system cannot be regarded asconstant, a densitometer 50 is coupled between manifold 35 or 40 andcontrol circuit 49, as depicted by phantom lines in FIG. 1A. In thiscase, the calculation of flow rate by control circuitry 49 takes intoaccount variations in specific gravity of the fluid, and the square rootof specific gravity becomes part of the flow rate determinativeparameter. In some cases, changes in specific gravity could be measuredindirectly by a thermometer, rather than by a densitometer.

For a description of the operation of the system disclosed in FIG. 1A,reference is made to FIG. 2 in which control circuitry 49 is disclosed,and to FIG. 4A in which local computer 11 for the embodiment of FIG. 1Ais disclosed. In FIG. 4A, samples of the analog output signal fromtransducer 48, which occur at a frequency determined by source 12, areapplied to an analog-to-digital converter 55 via input terminal 20. Eachheavy broken lead line represents four binary signal leads weightedaccording to the binary code. The output of analog-to-digital converter55, which represents the pressure difference between manifolds 35 and 40in the binary code, is coupled through contacts I and K of switch SW-1to a square root circuit 56. Analog-to-digital converter 55 is coupledthrough a contact J of switch SW-1 to output terminal 21 and inputterminal 22 is coupled through a contact L of switch SW-1 to square rootcircuit 56. The output of circuit 56, which represents the square rootof the pressure difference is coupled to a first input of a dividercircuit 57. The output of a register 58, which represents the squareroot of the specific gravity of the incompressible fluid being handled,is coupled to a second input of divider circuit 57. If densitometer 50is employed, its output is coupled through an analog-to-digitalconverter and a square root circuit (not shown) to register 58, toprovide an output that varies in accordance with the square root of thespecific gravity. The output of divider circuit 57, which represents thesquare root of the pressure difference divided by the specific gravity,is connected to a first input of a multiplier circuit 59. The output ofsequence circuit 19, which comprises the binary signals actuating plugs41, 42, 43 and 44 and is thus the binary number representative of thesum of the effective orifice areas of the open valve elements, isconnected via terminal 23 to a second input of multiplier circuit 59.The output of multiplier circuit 59, which represents the solution ofequation (1), i.e., the flow rate from manifold 35 to manifold 40, iscoupled via output terminal 24 to contacts A and D of switch SW-1.

When switches SW-1 and SW-2 are in the position shown, contacts A, C, E,G, I, K, and M are closed and the system operates in its set pointcontrol mode. Binary signals representing a set point value of flow rateare coupled from supervisory computer 15 to the second input ofcomparator 13 and to indicator 14. Responsive to the output ofcomparator 13, sequence circuit 19 counts up or down, thereby increasingor decreasing the flow rate through the valve elements until the binarysignals at output terminal 24 of computer 11 are identical to the binarysignals from supervisory computer 15. Then, the actual flow rate frommanifold 35 to manifold 40 is at the set point value, which is displayedon indicator 14 for monitoring purposes.

When switch SW-2 is placed in the other position, contact H is closedand the system operates in its manual control mode. Digital slew circuit16 is connected to the second input of comparator 13, and to indicator14. The binary output signals of circuit 16 change state in sequence sothey represent in turn each possible value of flow rate. When indicator14 displays the desired value of flow rate to be established, a humanoperator disables circuit 16. Then, sequence circuit 19 actuates thedigital valve elements to establish the actual flow rate from manifold35 to manifold 40 at this value, as described in the precedingparagraph.

When switch SW-1 is placed in the other position, contacts B, D, F, J,L, and N are closed and the system operates in its measurement mode. Theoutput of analog-to-digital converter 55 is connected through contact J(FIG. 4A) and contact F (FIG. 2) of switch SW-1 to the second input ofcomparator 13. The output of analog-to-digital converter 18 is connectedthrough contact B of switch SW-1 to the first input of comparator 13. Asthe flow rate from upstream manifold 35 to downstream manifold 40changes due to external conditions, the pressure difference betweenmanifolds 35 and 40 also changes. Sequence circuit 19 counts up or downresponsive to comparator 13 until the pressure difference indicated bytransducer 47 equals the magnitude of the signal from source 17. Thesignal magnitude of source 17 is adjusted to represent a sufficientlylarge pressure difference ΔP to cover the entire range of flow rates tobe measured. For example, it the fluid is water, the C_(v) when all thevalve elements are open is 15, and the maximum flow rate is 60 GPM, ΔPis 16 psi according to equation (1). Thus, the signal magnitude ofsource 17 is adjusted to equal the magnitude of the output signal fromtransducer 48 for a pressure difference of 16 psi. If the signalmagnitude of source 17 is too small, the valve elements are all openbefore the maximum flow rate is reached and the larger flow rates cannotbe measured. Moreover, if the fluid is very viscous, a dependence ofC_(v) upon ΔP results in a range of very low pressure differences sothis should be avoided by operating above such range. If the signalmagnitude of source 17 is too large, too few valve elements are openwhen the maximum flow rate is reached and the full resolving capacity ofthe measurement system is not utilized.

In FIG. 1B, is depicted another embodiment of a digital fluid flow ratemeasurement or control system, for an incompressible fluid, such aswater. This embodiment is useful when the minimum pressure difference isabout 10% of the upstream pressure or greater. The elements in commonwith the embodiment of FIG. 1A have the same reference numerals. Flowpassages 36, 37, 38, and 39 each have a cavitating venturi. Preferably,the valve body configuration and nozzle design disclosed in FIGS. 4 and5 of the Friedland and Langill application filed on even date herewithis employed. The resultant fluid flow rate from upstream manifold 35 todownstream manifold 40 through all the digital valve elements that arein the open state is expressed by the following equation: ##EQU2## whereQ is the mass flow rate of the incompressible fluid, C_(v) representsthe sum of the effective orifice areas of the open valve elements and isas defined above in connection with FIG. 1A, P_(g) is the gauge pressureof the fluid in manifold 35, P_(v) is the vapor pressure of the fluid,and SG is the specific gravity of the fluid. Since the pressure at thethroat of a cavitating venturi is the vapor pressure of the fluid,irrespective of pressure variations in downstream manifold 40, the flowrate is proportional to the product of the square root of the differencebetween the pressure in manifold 35 and the vapor pressure times the sumof the effective orifice areas of the digital valve elements in the openstate, and the flow rate can be computed therefrom for a specifiedfluid. Thus, the fluid flow rate determinative parameter is the squareroot of the pressure difference (P_(u) - P_(v)). The linear relationshipin equation (2) is maintained by the cavitating venturis, whicheliminate downstream vena contractas by virtue of the controlled fluiddivergence in the diverging sections of the venturis. As described inthe Friedland and Langill application filed on even data herewith, themaximum C_(v) of the system must be designed to be small enoughvis-a-vis the external "plumbing" to which the system is connected toprovide a minimum pressure difference between manifolds 35 and 40, i.e.,a pressure difference, when all the valve elements are in the openstate, sufficient to sustain cavitation at the throats of the venturis.Upstream manifold 35 is fluidically coupled to a gauge pressuretransducer 65, which generates an electrical analog signal proportionalto the guage pressure (P_(g)). This signal is coupled to control circuit49, which generates binary output signals that represent the actual flowrate (Q) in the binary code, to actuate plugs 41, 42, 43 and 44,respectively partially or wholly in response to the sensed fluidparameter, i.e., the square root of P_(g) - P_(v). Commericallyavailable gauge pressure transducers generate much less noise thancommerically available differential pressure transducers, so, otherfactors being equal, the embodiment of FIG. 1B is capable of measuringand controlling with greater accuracy than the embodiment of FIG. 1A. Itshould be noted that in the case of water, the vapor pressure isessentially zero pressure and the sensed fluid parameter simply is thesquare root of P_(g). If the specific gravity of the fluid in the systemcannot be regarded as constant, a densitometer 50 is coupled betweenmanifold 35 or 40 and control circuit 49, as depicted by phantom linesin FIG. 2A. In this case, the calculation of flow rate by controlcircuitry 49 takes into account variations in specific gravity of thefluid, and the square root of specific gravity becomes part of the flowrate determinative parameter.

FIG. 4B depicts local computer 11 for the embodiment of FIG. 1A. In FIG.4B, samples of the analog output signal from transducer 65, which occurat a frequency determined by source 12, are applied to ananalog-to-digital converter 66. Each heavy broken lead line representsfour binary signal leads weighted according to the binary code. Theoutput of analog-to-digital converter 66, which represents the gaugepressure in manifold 35 in the binary code, is coupled through contactsI and K of switch SW-1 to a first input of a difference circuit 67. Theoutput of analog-to-digital converter 66 is coupled through a contact Jof switch SW-1 to output terminal 21 and input terminal 22 is coupledthrough a contact L of switch SW-1 to the first input of differencecircuit 67. The output of a register 68, which represents the vaporpressure of the fluid being handled, is coupled to a second input ofdifference circuit 67. The output of difference circuit 67, whichrepresents the difference between the gauge pressure in manifold 35 andthe vapor pressure of the fluid, is connected to a square root circuit69. The output of square root circuit 69, which represents the squareroot of the pressure difference, is connected to a first input of adivider circuit 70. The output of a register 71, which represents thesquare root of the specific gravity of the fluid being handled, iscoupled to a second input of divider circuit 70. If densitometer 50 isemployed, its output is coupled through an analog-to-digital converterand a square root circuit (not shown) to register 71, to provide anoutput that varies in accordance with the square root of the specificgravity. The output of divider circuit 70, which represents the squareroot of the pressure difference divided by the specific gravity, isconnected to a first input of a multiplier circuit 72. The output ofsequence circuit 19, which comprises the binary signals actuating plugs41, 42, 43, and 44 and thus the binary number representing the sum ofthe effective orifice areas of the open valve elements, is connected viaterminal 23, to a second input of multiplier circuit 72. The output ofmultiplier circuit 72, which represents the solution of equation (2),i.e., the flow rate from manifold 35 to manifold 40, is coupled viaoutput terminal 24 to contacts A and D of switch SW-1 (FIG. 2).

In FIG. 1C is depicted a digital fluid flow rate measurement or controlsystem for a compressible fluid, such as air. The elements in commonwith the embodiment of FIG. 1A have the same reference numerals. Flowpassages 36, 37, 38, and 39 each have a critical flow orifice throughwhich the fluid flows at sonic velocity. Preferably, the configurationwith converging-diverging nozzle disclosed in FIG. 1 of the Friedlandand Langill application filed on even date herewith is employed. Theresultant fluid flow rate from upstream manifold 35 to downstreammanifold 40 through all the digital valve elements that are in the openstate is expressed by the following equation: ##EQU3## wherein w is themass flow rate of the compressible fluid, k is a constant depending onthe ratio of specific heats and the gas constant of the fluid, P_(A) isthe absolute static pressure of the fluid in manifold 35, T_(A) is theabsolute temperature of the fluid in manifold 35, A_(e) is the sum ofthe effective orifice areas of the open valve elements. Reference ismade to the text, The Dynamics and Thermodynamics of Compressible FluidFlow, by Ascher H. Shapiro, Vol. I, page 85, equation (4.17), The RonaldPress Co., N.Y. 1953, for the exact relationship between k, the ratio ofspecific heats, and the gas constant. The effective orifice area A_(e)of each individual valve element is the value yielded by equation (3)for given values of the other parameters when such valve element aloneis open, all other valve elements being closed. The flow rate isproportional to the absolute pressure in manifold 35 divided by thesquare root of the absolute temperature in manifold 35 times the productof the sum of the effective orifice areas of the digital valve elementsin the open state, and the flow rate can be computed therefrom for aspecified fluid. Thus, the fluid flow rate determinative parameter isthe absolute pressure (P_(A)) divided by the square root of the absolutetemperature (T_(A)). The linear relationship in equation (3) ismaintained by the critical flow orifices, which eliminate any dependenceof flow rate upon the pressure in downstream manifold 40. As describedin the Friedland and Langill application filed on even date herewith,the maximum A_(e) of the system must be designed to be small enoughvis-a-vis the external "plumbing" to which the system is connected toprovide a minimum pressure difference between manifolds 35 and 40 whenall the valve elements are in the open state sufficient to sustain fluidflow at sonic velocity through the critical flow orifices. Upstreammanifold 35 is fluidically coupled to a pressure transducer 75, whichgenerates an electrical analog signal proportional to the absolutepressure P_(A). Similarly, manifold 35 is thermally coupled to anabsolute temperature transducer 76, which generates an electrical analogsignal proportional to the absolute temperature T_(A). These signals arecoupled to control circuitry 49, which generates binary output signalsthat represent the actual flow rate (w') in the binary code, to actuateplugs 41, 42, 43 and 44, respectively, partially or wholly in responseto the sensed fluid parameter, i.e., P_(A) divided by the square root ofT_(A).

FIG. 4C depicts local computer 11 for the embodiment of FIG. 1C. In FIG.4C samples of the analog output signals from transducers 75 and 76,which occur at a frequency determined by source 12, are applied toanalog-to-digital converters 77 and 78, respectively. Each heavy brokenlead line represents four binary signal leads weighted according to thebinary code. The output of analog-to-digital converter 78, whichrepresents the absolute temperature of the fluid in manifold 35 in thebinary code, is coupled through a square root circuit 79 to a firstinput of a divider circuit 80. The output of analog-to-digital converter77, which represents the absolute pressure in manifold 35 in the binarycode, is coupled directly to a second input of divider circuit 80. Theoutput of divider circuit 80, which represents the absolute pressuredivided by the square root of the absolute temperature in manifold 35,is coupled through contacts I and K of switch SW-1 to a first input of amultiplier circuit 81. Divider circuit 80 is coupled through contact Jof switch SW-1 to output terminal 21 and input terminal 22 is coupledthrough contact L of switch SW-1 to the first input of multipliercircuit 81.

The output of a register 82, which represents the constant (k)characteristic of the particular compressible fluid, is coupled to asecond input of multiplier circuit 81. The output of multiplier circuit81, which represents the fluid constant times the absolute pressuredivided by the square root of the absolute temperature, is connected toa first input of a multiplier circuit 83. The output of sequence circuit19, which comprises the binary signals actuating plugs 41, 42, 43, and44 and thus the binary number representing the sum of the effectiveorifice areas of the open valve elements, is connected via terminal 23to a second input of multiplier circuit 83. The output of multipliercircuit 83, which represents the solution of equation (3), i.e., theflow rate from manifold 35 to manifold 40, is coupled via outputterminal 24 to contacts A and D of switch SW-1.

In some cases the available pressure is not sufficient to sustainoperation of critical flow orifices in the embodiment of FIG. 1C. Thusthe linear relationship of equation (3) cannot be maintained. In thiscase, an absolute pressure transducer 93 is coupled between manifold 40and control circuitry 49, as represented by the phantom lines in FIG.1C. This provides an additional input to establish the non-linearrelationship between the flow rate and the product of the flow ratedeterminative parameter and the states of the valve elements. FIG. 4D isan alternative version of a portion of local computer 11 shown in FIG.4C. The output of transducer 75 is coupled through an analog-to-digitalconverter 94 to one input of a divider circuit 95. The output oftransducer 93 is coupled through an analog-to-digital converter 96 tothe other input of divider circuit 95. The outputs of transducers 75 and93 are sampled at a rate determined by source 12. Each heavy broken leadline represents four binary signal leads weighted according to thebinary code. The output of divider circuit 95, which represents thepressure in the downstream manifold divided by the pressure in theupstream manifold, is connected to the input of a function generator 97.Function generator 97 introduces the well known non-linearityrepresented by the subsonic portion of the curve in FIG. 4.3 on page 76of the Shapiro text referenced above. In other words, for an inputhaving a particular value on the abscissa of the curve, the output fromfunction generator 97 has the value of the ordinant of the curve. Theoutput of function generator 97 and the output of register 82 arecoupled to respective inputs of multiplier circuit 81 for multiplicationwith a signal representative of the flow rate determinative parameter,i.e., the absolute stagnation pressure (P_(A)) divided by the squareroot of the absolute stagnation temperature (T_(A)). The output ofmultiplier 81 is processed in the manner described above in connectionwith FIG. 4C, to produce a flow rate representative signal that reflectsthe nonlinear relationship.

The embodiments of FIGS. 1B and 1C operate in the set point control,manual control, and measurement modes in the same manner described abovein connection with the embodiment of FIG. 1A.

In FIG. 5, several functions of control circuitry 49 in FIG. 2 aremonitored and displayed on a digital indicator 90. When a contact A of aselector switch SW-3 is closed, indicator 90 displays the value of thefirst input to comparator 13. When a contact B of switch SW-3 is closed,indicator 90 displays the difference in value between the first andsecond inputs to comparator 13. When a contact C of switch SW-3 isclosed, indicator 90 displays the output of sequence circuit 19.

The ratio of the effective cross-sectional orifice areas of the valveelements could be weighted according to any binary code; for example,they could be weighted to follow a straight geometric progression oftwo, they could be weighted equally, or they could be weighted so thesmaller valve elements follow a geometric progression of two and thelarger valve elements are equal. In any case, the binary signalsactuating the valve elements are weighted according to the same binarycode as the respective valve elements to which they are coupled.

The flow rate measurement or control range of the system can beincreased without impairing accuracy by simply adding more valveelements; for example, assuming the valve elements are weightedaccording to a straight binary progression of two, eight valve elementsprovide an operating range of 0 to 256 units, 10 valve elements providean operating range of 0 to 1,024 units, and 12 valve elements provide anoperating range of 0 to 4,096 units, where the unit is determined by theC_(r) or A_(e) of the valve element with the smallest value in thebinary code. In terms of resolution, eight valve elements provide aresolution of 0.4%, 10 valve elements provide a resolution of 0.1%, and12 valve elements provide a resolution of 0.025%. Accuracy is notimpaired as the operating range is expanded because the flow rateremains proportional to the product of the sensed fluid parameter timesthe sum of the effective orifice areas of the open valve elements. Ineffect, viewing the invention as an orifice meter, if eight valveelements are employed, there are 255 separate orifice plates, each witha different size orifice; each orifice plate becomes effective at anappropriate flow rate within the range.

The described embodiments of the invention are only considered to bepreferred and illustrative of the invention concept; the scope of theinvention is not to be restricted to such embodiments. Various andnumerous other arrangements may be devised by one skilled in the artwithout departing from the spirit and scope of this invention. Forexample, the calculations performed by local computer 11 could becarried out by an analog computer instead of the digital computingcircuits depicted in detail in FIGS. 4A, 4B, and 4C or by a programmeddigital computer.

Reference is made to the calibration procedure described on pages 11through 13 of a copending sole application Ser. No. 432,152 of Robert A.Gallatin, filed on even date herewith and assigned to the assignee ofthe present application by an assignment recorded on even date herewith.The effective orifice area (C_(v) or A_(e)) of each valve element isindividually adjusted in the manner described in the sole Gallatinapplication to establish the correct value weighting.

What is claimed is:
 1. A digital fluid flow rate measurement or controlsystem comprising:a source of fluid at a first pressure; a fluidreceiver at a second pressure lower than the first pressure; a pluralityof individually actuatable, value weighted digital bistable valveelements interconnecting the source to the receiver, each valve elementassuming exclusively either an open state in which fluid flows from thesource through the valve element to the receiver or a closed state inwhich no fluid flows from the source through the valve element to thereceiver such that the resultant fluid flow rate from the source to thereceiver is a function of the product of a flow rate determinative fluidparameter times the sum of the weighted values of the digital valveelements in the open state; means for sensing the fluid parameter andgenerating a first signal representative of the value of the fluidparameter; means responsive to the value of the fluid parameter and thestates of the digital valve elements for generating a second signalrepresentative of the value of the resultant fluid flow rate; and meansresponsive to one of the signals for controlling the states of thedigital valve elements so as to maintain constant the value representedby the one signal.
 2. The system of claim 1, in which the fluid isincompressible, the fluid parameter is the square root of the differencebetween the first and second pressures, and the sensing means senses thesquare root of the difference between the first and second pressures. 3.The system of claim 1, in which the fluid is incompressible, eachdigital valve element has a passage from the source to the receiverwhich includes a converging-diverging nozzle designed to maintain thevapor phase of the fluid at its throat, the fluid parameter is thesquare root of the difference between the first pressure and the vaporpressure of the fluid, and the sensing means senses the first pressure.4. The system of claim 1, in which the fluid is compressible, the fluidparameter is the absolute stagnation pressure divided by the square rootof the absolute stagnation temperature, the sensing means senses thefirst pressure and the temperature of the fluid at the source, eachdigital valve element has a flow passage from the source to thereceiver, a flow determining orifice formed in the flow passage, and aregion in the flow passage through which the fluid flows at sonicvelocity thereby isolating the flow passage upstream of the flowdetermining orifice from variations in the second pressure.
 5. Thesystem of claim 1, in which the fluid is compressible, the fluidparameter is the first pressure divided by the square root of theabsolute stagnation temperature, and the sensing means senses the firstpressure, the second pressure, and the temperature of the fluid at thesource or receiver.
 6. The system of claim 1, in which the controllingmeans comprises means for controlling the states of the valve elementsto maintain the value of the fluid parameter constant.
 7. The system ofclaim 6, in which the controlling means additionally comprises:a sourceof a third signal proportional to a desired fluid flow rate from thesource to the receiver, and means responsive to the difference betweenthe third and second signals for changing the states of the digitalvalve elements to reduce such difference.
 8. The system of claim 6,additionally comprising an indicator responsive to the controlling meansfor displaying the states of the valve elements as a measurement of thefluid flow rate.
 9. The system of claim 1, in which the controllingmeans comprises:a source of a third signal proportional to a desiredfluid flow rate from the source to the receiver, and means responsive tothe difference between the third and second signals for changing thestates of the digital valve elements to reduce such difference.
 10. Thesystem of claim 9, in which the values of the respective digital valveelements are weighted according to a binary code, the third signalcomprises a plurality of binary signals equal in number to the digitalvalve elements and weighted according to the binary code, and the secondsignal comprises a plurality of binary signals equal in number to thedigital valve elements and weighted according to the binary code. 11.The system of claim 1, in which the fluid flow rate through at leastsome of the respective digital valve elements in the open state areweighted according to a geometric progression of two.
 12. The system ofclaim 1, additionally comprising means for maintaining a linear functionrelationship between the resultant fluid flow rate and the product. 13.The system of claim 12, in which the valve elements are arranged so thefluid streams flowing through them are directed at each other.
 14. Thesystem of claim 1, in which the one signal is the first signal and thevalue of the fluid parameter is maintained constant by the controllingmeans.
 15. The system of claim 1, in which the one signal is the secondsignal and the value of the resultant fluid flow rate is maintainedconstant by the controlling means.
 16. The system of claim 1,additionally comprising switching means for alternatively applying tothe controlling means the first signal to measure flow rate or thesecond signal to control flow rate.
 17. The system of claim 1, in whichthe second signal generating means comprises a signal multiplierresponsive to the first signal and the states of the digital valveelements.
 18. The system of claim 17, in which the sensing and firstsignal generating means comprises transducer means for generating asignal proportional to the fluid parameter and a function generator formodifying the signal generated by the transducer means.
 19. A digitalliquid flow rate measurement or control device comprising:an upstreamliquid manifold; a downstream liquid manifold; a plurality of at leastthree individually actuatable, digital valve elements, each valveelement having a liquid flow passage leading from the upstream manifoldto the downstream manifold, a sealable orifice in the passage, and abistable plug positionable exclusively in a first state in which theplug seals the orifice to prevent liquid flow through the passage or asecond state in which the plug unseals the orifice to permit liquid flowthrough the passage; first liquid pressure sensing means located in theupstream manifold; second liquid pressure sensing means located in thedownstream manifold; and means responsive to the liquid pressuredifference sensed by the first and second pressure sensing means and thestates of the digital valve elements for generating a signalrepresentative of the resultant fluid flow rate from the upstreammanifold to the downstream manifold.
 20. The device of claim 19,additionally comprising means for controlling the states of therespective plugs to maintain the liquid pressure difference sensed bythe first and second pressure sensing means at a substantially constantvalue.
 21. The device of claim 20, additionally comprising an indicatorresponsive to the resultant fluid flow rate representative signal fordisplaying the liquid flow rate from the upstream manifold to thedownstream manifold.
 22. The device of claim 19, additionallycomprising:a source of a set point signal proportional to a desiredliquid flow rate from the upstream manifold to the downstream manifold,and means responsive to the difference between the resultant flow raterepresentative and set point signals for changing the plug states of therespective digital valve elements to equalize the desired flow rate andthe actual flow rate.
 23. The device of claim 19, additionallycomprising means for maintaining a linear relationship between theresultant flow rate through the valve elements in the second positionand the product of the sensed pressure difference times the sum of theeffective cross-sectional areas of the valve elements in the secondstate.
 24. A digital liquid flow measurement or control devicecomprising:an upstream liquid manifold; a downstream liquid manifold; aplurality of at least three individually actuatable, digital valveelements, each valve element having a liquid flow passage leading fromthe upstream manifold to the downstream manifold, a sealable orifice inthe passage, a bistable plug positioned exclusively in a first state inwhich the plug seals the orifice to prevent liquid flow through thepassage or a second state in which the plug unseals the orifice topermit liquid flow through the passage, and a cavitating venturi in thepassage at which the liquid remains in its vapor phase when the plug isin the second state; liquid pressure sensing means located in theupstream manifold; and means reponsive to the liquid pressure sensed bythe pressure sensing means and the states of the digital valve elementsfor generating a signal representative of the resultant fluid flow ratefrom the upstream manifold to the downstream manifold.
 25. The device ofclaim 24, additionally comprising means for controlling the states ofthe respective plugs to maintain the liquid pressure sensed by thepressure sensing means at a substantially constant value.
 26. The deviceof claim 25, additionally comprising an indicator responsive to theresultant flow rate representative signal for displaying the liquid flowrate from the upstream manifold to the downstream manifold.
 27. Thedevice of claim 24, additionally comprising:a source of a set pointsignal proportional to a desired liquid flow rate from the upstreammanifold to the downstream manifold, and means responsive to thedifference between the resultant flow rate representative and set pointsignals for changing the plug states of the respective digital valveelements to equalize the desired flow rate and the actual flow rate. 28.A digital gas flow measurement or control device comprising:an upstreamgas manifold; a downstream gas manifold; a plurality of at least threeindividually actuatable, digital valve elements, each valve elementhaving a gas flow passage leading from the upstream manifold to thedownstream manifold, a sealable orifice in the passage, a bistable plugpositionable exclusively in a first state in which the plug seals theorifice to prevent gas flow through the passage or a second state inwhich the plug unseals the orifice to permit gas flow through thepassage, and a critical flow orifice through which gas flow at sonicvelocity is maintained when the plug is in the second state; absolutegas pressure sensing means located in the upstream manifold; absolutetemperature sensing means located in the upstream manifold; and meansresponsive to the absolute pressure sensed by the pressure sensing meansdivided by the square root of the absolute temperature sensed by thetemperature sensing means and the states of the digital valve elementsfor generating a signal representative of the resultant fluid flow ratefrom the upstream manifold to the downstream manifold.
 29. The device ofthe claim 28, additionally comprising means for controlling the statesof the respective plugs to maintain the absolute pressure sensed by thepressure sensing means divided by the square root of the absolutetemperature sensed by the temperature sensing means at a substantiallyconstant value.
 30. The device of claim 29, additionally comprising anindicator responsive to the resultant flow rate representative signalfor displaying the gas flow rate from the upstream manifold to thedownstream manifold.
 31. The device of claim 28, additionallycomprising:a source of a set point signal proportional to a desired gasflow rate from the upstream manifold to the downstream manifold, andmeans responsive to the difference between the resultant flow raterepresentative and set point signals for changing the plug states of therespective digital valve elements to equalize the desired flow rate andthe actual flow rate.
 32. The device of claim 28, in which the criticalflow orifice comprises the throat of a converging-diverging nozzle. 33.A digital gas flow measurement or control device comprising:an upstreamgas manifold; a downstream gas manifold; a plurality of at least threeindividually actuatable, digital valve elements, each valve elementhaving a gas flow passage leading from the upstream manifold to thedownstream manifold, a sealable orifice in the passage, and a bistableplug positionable exclusively in a first state in which the plug sealsthe orifice to prevent gas flow through the passage or a second state inwhich the plug unseals the orifice to permit gas flow through thepassage; first gas pressure sensing means located in the upstreammanifold; second gas pressure sensing means located in the downstreammanifold; temperature sensing means located in one of the manifolds; andmeans responsive to the pressures sensed by the first and secondpressure sensing means and the temperature sensed by the temperaturesensing means and the states of the digital valve elements forgenerating a signal representative of the resultant fluid flow rate fromthe upstream manifold to the downstream manifold.
 34. The device ofclaim 33, additionally comprising means for controlling the states ofthe respective plugs to maintain the pressure sensed by the firstpressure sensing means divided by the square root of the absolutetemperature sensed by the temperature sensing means at a substantiallyconstant value.
 35. The device of claim 34, additionally comprising anindicator responsive to the resultant flow rate representative signalfor displaying the gas flow rate from the upstream manifold to thedownstream manifold.
 36. The device of claim 33, additionallycomprising:a source of a set point signal proportional to a desired gasflow rate from the upstream manifold to the downstream manifold, andmeans responsive to the difference between the resultant flow raterepresentative and set point signals for changing the plug states of therespective digital valve elements to equalize the desired flow rate andthe actual flow rate.
 37. A method of measuring the flow rate in a fluidline between a source of fluid at a first pressure and a fluid receiverat a second pressure lower than the first pressure, the methodcomprising the steps of:interconnecting a plurality of individuallyactuatable, value weighted digital bistable valve elements in parallelin the fluid line between the source and the receiver, each valveelement assuming exclusively either an open state in which fluid flowsfrom the source through the valve element to the receiver or a closedstate in which no fluid flows from the source through the valve elementto the receiver such that the resultant flow rate through the fluid lineis a function of the product of a flow rate determinative fluidparameter times the sum of the weighted values of the digital valveelements in the open state; sensing the .Iadd.flow rate determinative.Iaddend.fluid parameter; controlling the states of the digital valveelements to maintain .[.the.]. .Iadd.a .Iaddend.fluid .[.parameter.]..Iadd.characteristic in the line .Iaddend.constant .[.as the flow ratethrough the fluid line varies..]..Iadd.; sensing the digital valveelements in the open state; and indicating the resultant flow ratethrough the fluid line from the sensed flow rate determinative fluidparameter and the sensed digital valve elements in the open state..Iaddend.
 38. The method of claim 37, additionally comprising the stepof maintaining a linear function relationship between the resultant flowrate through the valve elements in the open state and the product.
 39. Adigital fluid flow rate measurement or control system comprising:asource of fluid at a first pressure; a fluid receiver at a secondpressure lower than the first pressure; a plurality of individuallyactuatable, value weighted digital bistable valve elementsinterconnecting the source to the receiver, each valve element assumingexclusively either an open state in which fluid flows from the sourcethrough the valve element to the receiver or a closed state in which nofluid flows from the source through the valve element to the receiver;means for maintaining a linear relationship between the resultant fluidflow rate from the source to the receiver and the product of a flow ratedeterminative fluid parameter times the sum of the weighted values ofthe digital valve elements in the open state; means for sensing thefluid parameter; and means responsive to the value of the fluidparameter and the states of the digital valve elements for generating asignal representative of the resultant fluid flow rate.
 40. The systemof claim 39, additionally comprising means for controlling the states ofthe valve elements to maintain the value of the fluid parameterconstant.
 41. The system of claim 39, in which the generating means isresponsive to the sensed value of the fluid parameter and the states ofthe digital valve elements, the system additionally comprising means forcontrolling the states of the digital valve elements responsive to thesignal representative of the resultant fluid flow rate.
 42. The systemof claim 39, in which the fluid is incompressible, the fluid parameteris the square root of the difference between the first and secondpressures, the sensing means senses the square root of the differencebetween the first and second pressures, and the means for maintaining alinear relationship comprises means for establishing a sufficiently lowmaximum difference between the first and second pressures to preventformation of vena contractas having pressure dependent cross-sectionalareas downstream of the valve elements in the open state.
 43. The systemof claim 39, in which the fluid is incompressible, the fluid parameteris the square root of the difference between the first and secondpressures, the sensing means senses the square root of the differencebetween the first and second pressures, and the means for maintaining alinear relationship comprises means for directing the streams from thevalve elements in the open state at each other to dissipate the venacontractas.
 44. The system of claim 39, in which the fluid isincompressible, the fluid parameter is the square root of differencebetween the first pressure and the vapor pressure of the fluid, thesensing means senses the first pressure, and the means for maintaining alinear relationship comprises a cavitating venturi in each valveelement.
 45. The system of claim 39, in which the fluid is compressible,the fluid parameter is the absolute stagnation pressure divided by thesquare root of the absolute stagnation temperature, the sensing meanssenses the first pressure and the temperature of the fluid at thesource, and the means for maintaining a linear relationship comprises acritical flow orifice in each valve element through which fluid flows atsonic velocity.
 46. The system of claim 45, in which the orifice is thethroat of a converging-diverging nozzle.
 47. A method for operating adigital fluid flow control system having a plurality of individuallyactuatable, value weighted digital bistable valve elementsinterconnecting a source of fluid at a first pressure to a receiver at asecond pressure lower than the first pressure, each valve elementassuming exclusively either an open state in which fluid flows from thesource through the valve element to the receiver or a closed state inwhich no fluid flows from the source through the valve element to thereceiver such that the resultant fluid flow rate from the source to thereceiver is a function of the product of a flow rate determining fluidparameter times the sum of weighted values of the digital valve elementsin the open state, the method comprising the steps of:sensing the fluidparameter and generating a first signal representative of the value ofthe fluid parameter; generating a second signal representative of theproduct of the value of the fluid parameter and the states of thedigital valve elements; and controlling the states of the digital valveelements responsive to one of the signals so as to maintain constant thevalue representated by the one signal. .Iadd.
 48. A digital fluidcontrol system comprising: a first fluid manifold; a second fluidmanifold; a plurality of individually actuatable, value weighted digitalbistable valve elements interconnecting the first manifold to the secondmanifold, each valve element assuming exclusively either an open statein which fluid flows from the first manifold through the valve elementto the second manifold or a closed state in which no fluid flows fromthe first manifold through the valve element to the second manifold suchthat the resultant fluid flow rate from the first manifold to the secondmanifold is a function of the product of a flow rate determinative fluidparameter times the sum of the weighted values of the digital valveelements in the open state; means responsive to the value of the flowrate determinative fluid parameter and the states of the digital valveelements for generating a signal representative of the value of theresultant fluid flow rate; means for sensing a control parameter in thesystem; and means responsive to the sensed control parameter forcontrolling the states of the digital valve elements so as to maintainthe sensed control parameter constant. .Iaddend..Iadd.
 49. The controlsystem of claim 48, in which the sensed control parameter is thepressure in one of the manifolds. .Iaddend. .Iadd.
 50. The controlsystem of claim 49, additionally comprising an indicator responsive tothe signal representative of the value of the resultant fluid flow rate..Iaddend..Iadd.
 51. The control system of claim 50, in which the fluidflow rate determinative parameter and the sensed control parameter areidentical, the generating means being responsive to the sensing means..Iaddend..Iadd.
 52. The control system of claim 51, in which the fluidis incompressible, each valve element has a passage from the firstmanifold to the second manifold which includes a converging-divergingnozzle designed to maintain the vapor phase of the fluid at its throat,the fluid parameter is the square root of the difference between thepressure in the first manifold and the vapor pressure of the fluid, andthe sensing means senses the pressure in the first manifold..Iaddend..Iadd.
 53. The control system of claim 48, additionallycomprising an indicator responsive to the signal representative of thevalue of the resultant fluid flow rate. .Iaddend..Iadd.
 54. The controlsystem of claim 48, in which the fluid flow rate determinative parameterand the sensed control parameter are identical, the generating meansbeing responsive to the sensing means. .Iaddend. .Iadd.
 55. A digitalfluid control system comprising: a source manifold; a receiver manifold;a plurality of individually actuatable, value weighted digital bistablevalve elements interconnecting the source manifold to the receivermanifold, each valve element assuming exclusively either an open statein which fluid flows from the source manifold through the valve elementto the receiver manifold or a closed state in which no fluid flows fromthe source manifold through the valve element to the receiver manifoldsuch that the resultant fluid flow rate from the source manifold to thereceiver manifold is a function of the product of a flow ratedeterminative fluid parameter times the sum of the weighted values ofthe digital valve elements in the open state; means for sensing the flowrate determinative fluid parameter; means responsive to the sensed flowrate determinative fluid parameter and the states of the digital valveelements for generating a signal representative of the value of theresultant fluid flow rate; and means for controlling the states of thedigital valve elements so as to maintain the value of a fluidcharacteristic in the system constant. .Iaddend..Iadd.
 56. The controlsystem of claim 55, in which the fluid characteristic is the pressure inone of the manifolds. .Iaddend. .Iadd.
 57. The control system of claim56, additionally comprising an indicator responsive to the signalrepresentative of the value of the resultant fluid flow rate..Iaddend..Iadd.
 58. The control system of claim 55, in which the fluidis incompressible, the flow rate determinative fluid parameter is thesquare root of the difference between the pressures in the sourcemanifold and the receiver manifold, and the sensing means senses thesquare root of said pressure difference. .Iaddend..Iadd.
 59. The controlsystem of claim 55, in which the fluid is incompressible, each digitalvalve element has a passage from the source to the receiver whichincludes a converging-diverging nozzle designed to maintain the vaporphase of the fluid at its throat, the flow rate determinative fluidparameter is the square root of the difference between the pressure inthe source manifold and the vapor pressure of the fluid, and the sensingmeans senses the pressure in the source manifold. .Iaddend. .Iadd. 60.The control system of claim 55, in which the fluid is compressible, theflow rate determinative fluid parameter is the absolute stagnationpressure divided by the square root of the absolute stagnationtemperature, the sensing means senses the pressure and the temperatureof the fluid in the source manifold, each digital valve element has aflow passage from the source manifold to the receiver manifold, a flowdetermining orifice formed in the flow passage, and a region in the flowpassage through which the fluid flows at sonic velocity therebyisolating the flow passage upstream of the flow determining orifice fromvariations in the pressure in the receiver manifold. .Iaddend..Iadd. 61.The method of claim 37, in which the fluid characteristic is the flowrate determinative fluid parameter. .Iaddend. .Iadd.
 62. A method foroperating a digital fluid flow control system having a plurality ofindividually actuatable, value weighted digital bistable valve elementsinterconnecting a source of fluid at a first pressure to a receiver at asecond pressure lower than the first pressure, each valve elementassuming exclusively either an open state in which fluid flows from thesource through the valve element to the receiver or a closed state inwhich no fluid flows from the source through the valve element to thereceiver such that the resultant fluid flow rate from the source to thereceiver is a function of the product of a flow rate determinative fluidparameter times the sum of the weighted values of the digital valveelements in the open state, the method comprising the steps of: sensingthe flow rate determinative fluid parameter; generating a signalrepresentative of the product of the value of the flow ratedeterminative fluid parameter and the states of the digital valveelements; and controlling the states of the digital valve elements so asto maintain the value of a fluid characteristic in the system constant..Iaddend.